On an un-named war-torn planet, an injured soldier named John Francis Vater (Joe Anderson) is hunted by a robot ambulance. It kills him and he is turned into a cylinder with an AI personality. The Doctor (Ncuti Gatwa) and companion Ruby Sunday (Millie Gibson) are alerted by Vater’s screams. The Doctor steps on a landmine and cannot move. The mine, made by the Villergard Weapons Company, alters the DNA of anyone who steps on it and turns them into a bomb. The DNA of a Timelord might destroy half the planet.
Ruby gives him Vater’s cylinder to maintain his balance. Vater’s daughter Splice (Caolinn Springall), looking for her father, comes on the scene. Ruby keeps her away so she will not trigger the mine. They are joined by a soldier named Mundy Flynn (Varada Sethu) who tells them the Anglican Army is fighting the alien Kastarions. Mundy shoots the Doctor and another ambulance arrives. Ruby and Mundy try to distract it so it will not treat the Doctor. Another soldier, Canterbury James Oliphant, or Canto, arrives and develops a crush on Mundy and shoots Ruby.
The Doctor realizes that the Kastarion enemy does not really exist and it is the Villergard Weapons Company who is prosecuting the war. He convinces Vater’s AI to prove it to the army. Canto is killed trying to configure an ambulance to treat Ruby. Canto’s AI admits to loving Mundy, though the ambulances try to prevent it. Vater succeeds in stopping the war, which allows the Doctor to step off the landmine and the ambulance to give Ruby genuine medical treatment. Mundy takes in Splice, and the Doctor and Ruby depart.
The episode was written by Steven Moffat, long a writer for Doctor Who, and directed by Julie Ann Robinson. The war encampment was built on a soundstage instead of using a real crater, because the latter was out in the weather. The AI was voiced by Susan Twist. The episode garnered 88% on Rotten Tomatoes, and was called simple, audacious, and exhilarating, though some reviewers were not convinced by the love-story.